This application includes subject-matter incorporated from applicant""s British Patent Application Serial No. 9814123.7 filed on Jul. 1, 1998.
The present invention relates to electrochemical cells and particularly to fuel cells incorporating a proton exchange membrane. More particularly, the present invention relates to the use of printed circuit boards to form internal separator layers for non-planar electrolyte layered fuel cells.
Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. In electrochemical fuel cells employing hydrogen as the fuel and oxygen as the oxidant, the reaction product is water. Conventional proton exchange membrane (xe2x80x9cPEMxe2x80x9d) fuel cells generally employ a planar, layered structure known as a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d), comprising a solid polymer electrolyte or ion exchange membrane, which is neither electrically conductive nor porous, disposed between an anode electrode layer and a cathode electrode layer. The electrode layers are typically comprised of porous, electrically conductive sheets with elecro-catalyst particles at each membrane-electrode interface to promote the desired electrochemical reaction.
During operation of the fuel cell, hydrogen from a fuel gas stream moves from fuel channels through the porous anode electrode material and is oxidized at the anode electro-catalyst to yield electrons to the anode plate and hydrogen ions which migrate through the electrolyte membrane. At the same time, oxygen from an oxygen-containing gas stream moves from oxidant channels through the porous electrode material to combine with the hydrogen ions that have migrated through the electrolyte membrane and electrons from the cathode plate to form water. A useful current of electrons travels from the anode plate through an external circuit to the cathode plate to provide electrons for the reaction occurring at the cathode electro-catalyst.
In conventional fuel cells, the MEA is interposed between two rigid, planar, substantially fluid-impermeable, electrically conductive plates, commonly referred to as separator plates. The plate in contact with the anode electrode layer is referred to as the anode plate and the plate in contact with the cathode electrode layer is referred to as the cathode plate. The separator plates (1) serve as current collectors, (2) provide structural support for the MEA, and (3) typically provide reactant channels for directing the fuel and oxidant to the anode and cathode electrode layers, respectively, and for removing products, such as water, formed during operation of the fuel cell. Fuel channels and oxidant channels are typically formed in the separator plates; the plates are then normally referred to as fluid flow field plates. Herein, xe2x80x9cfluidxe2x80x9d shall include both gases and liquids; although the reactants are typically gaseous, the products may be liquids or liquid droplets as well as gases.
Multiple unitary fuel cells can be stacked together to form a conventional fuel cell stack to increase the overall power output. Stacking is typically accomplished by the use of electrically conductive bipolar plates which act both as the anode separator plate of one fuel cell and as the cathode separator plate of the next fuel cell in the stack. One side of the bipolar plate acts as an anode separator plate for one fuel cell, while the other side of the bipolar plate acts as a cathode separator plate for the next fuel cell in the stack. The bipolar plates combine the functions of anode and cathode plates referred to above and are provided with the fuel channels and oxidant channels. The internal structure of fuel cell stacks based on planar MEA elements requires complex bi-polar separator plates in which the fluid flow channels have been formed by removing material from the plate, usually through some form of machining process.
Watkins, U.S. Pat. Nos. 4,988,583 and 5,108,849, issued Jan. 29, 1991 and Apr. 28, 1992, respectively, describe fluid flow field plates in which continuous open-faced fluid flow channels formed in the surface of the plate traverse the central area of the plate surface in a plurality of passes, that is, in a serpentine manner, between an inlet manifold opening and an outlet manifold opening formed in the plate. These patents are typical of conventional fuel cell designs.
Undulate electrolyte layer fuel cells have also been proposed in high temperature, molten carbonate type fuel cells. For example, BBC Brown Boveri (FR 2306540) proposes a non-planar electrolyte layered molten carbonate fuel cell, and German Patent DE 3812813 proposes the use of a non-planar glass electrolyte layer. Japanese patent 1-292759 takes the non-planar electrolyte molten carbonate fuel cell concept one step further, proposing a different means of obtaining the non-planar structure. These molten carbonate cells are based entirely upon the use of planar separator layers and rely exclusively upon the use of metals and high temperature bonding techniques for cell construction. Construction of a PEM cell is impossible using the concepts disclosed in these patents.
McIntyre, U.S. Pat. No. 4,826,554, issued May 2, 1989, discloses a sinuously-formed xe2x80x9celectrically conductive, hydraulically permeable matrix 130, which is also embedded into the membrane sheet 120xe2x80x9d. However, there is no disclosure of alternating layers in a stack that contact one another to form interior flow conduits or channels.
Japanese Patent Publication No. 50903/1996, Futoshi et al., Feb. 20, 1996, discloses a solid polymer fuel cell having generally planar separators with alternating protruding parts serving to clamp a power generation element (apparently an MEA) into a non-planar but piecewise linear shape. The area of the MEA exposed to reactants is increased relative to planar MEA designs, but the portions of the MEA clamped between the protruding parts and the planar portion of each separator do not appear to be exposed to reactants. Further, significant clamping force appears to be required to reduce contact resistance. Such force, together with the abrupt changes in direction at the corners of the protruding parts, may introduce kinks and very large stresses in the MEA.
Separators that have been disclosed in the prior art are typically composed of flat sheets of simply conductive material such as metal or in some cases graphite.
British application Serial No. 9814123.7 (McLean et al., assigned to the applicant herein) filed on Jul. 1, 1998 and derivatives and divisionals thereof provide details of different aspects of non-planar MEA layers in PEM fuel cells, and other aspects of PEM fuel cell design.
In accordance with the present invention, a fuel cell stack comprises a stacked series of MEA structures alternating with aligned separator plates, each MEA structure being non-planar and having sufficient rigidity to retain its shape when the stack is placed under sufficient pressure in the stacking direction to maintain physical and electrical contact between each MEA structure and the adjacent separator plate and forming ,thereby, the fuel and oxidant channels between the MEA structure and the separator plates, each separator plate comprising an electrically insulating substrate overlaid on each surface by a selected pattern of electrically conductive traces, each trace on one surface of the substrate electrically connected to at least one trace on the opposite surface of the substrate by a conductive path, and the pattern of the traces selected so that the traces on each surface of the substrate are in electrical contact with the adjacent MEA structure in the fuel cell stack when the separator plate is aligned with the adjacent MEA structures and stacked in the fuel cell stack.
By employing non-planar MEA separator layers it becomes possible to build up a complex flow-field fuel cell stack by forming sheet elements into three dimensional structures based on periodic undulating waveforms. The resulting fuel cell stack can be manufactured in a continuous process with virtually no waste material. The resulting stack also has higher power density than its conventional counterpart since the MEA layer is undulate and therefore covers a larger surface area in the same volume and since layers can be stacked together more tightly than in the planar MEA case through the use of undulate separators in addition to the undulate MEA layers.
By forming the fuel and oxidant channels by the separation of the non-planar MEA structure from the separator plates a greater portion of the MEA structure is exposed to the fuel and oxidant fluids as compared to prior structures where the channels are formed by the separation of a non-planar separator plate from a planar MEA structure.
In accordance with our invention, which incorporates a non-planar electrolyte layer, the required separator function is provided by a composite separator plate of a non-conducting material with conductive traces formed on its surface by printed circuit processes which may be incorporated in non-planar MEA layer fuel cell stacks employing either planar or non-planar separator layers.
The use of printed circuit board technology in the manufacture of separator strata is advantageous because it is possible to create traces onto which the screens can be soldered that are very narrow transversely. Narrow traces minimize the amount of exposed metal as the traces are metallic; exposed metal can shorten cell lifetimes in two ways. First, the metal itself may corrode, which can cause premature failure of the structure. Second, metal ions may be deposited into the catalyst layer, causing catalyst sites to become xe2x80x9ccloggedxe2x80x9d with metal ions and ultimately xe2x80x9cchokingxe2x80x9d the cell.
Further, narrow traces allow for simple alignment of the screens in the manufacturing stage by the use of the surface tension of molten solder to draw the screens into alignment when the screens are soldered in place by re-flow soldering a process commonly used in the assembly of surface mount components on printed circuit boards.
It is also an aspect of the invention to use hydrogen in a fuel cell stack made up of fuel cells having separator plates as heretofore described and connectable via an anode terminal and a cathode terminal to an external load. Each fuel cell has an MEA layer and two discrete associated reactant-gas impermeable separator layers. The MEA layer has a porous anode electrode, a porous cathode electrode, an electrolytic membrane layer disposed between the two electrodes, an anode electro-catalyst layer disposed between the electrolytic membrane layer and the anode electrode, and a cathode electro-catalyst layer disposed between the electrolytic membrane layer and the cathode electrode. One side of one separator layer in conjunction with the MEA layer provides at least one flowpath of a flow field for hydrogen and one side of the other separator layer in conjunction with the MEA layer provides at least one flowpath of a flow field for a selected oxidant. The flowpaths are constituted over their greater length by parallel transversely spaced and longitudinally extending flow channels interconnected in the vicinity of their ends to form the flowpaths. The MEA layer is installed in the stack between the associated separator layers so that the side of the separator layer that in conjunction with the MEA layer provides flow channels of a flow field for hydrogen faces and is in contact with the anode side of the MEA layer, whilst the side of the separator layer providing flow channels of a flow field for oxidant faces and is in contact with the cathode side of the MEA layer, so that the hydrogen flow channels are closed to form a conduit for supplying hydrogen to the MEA layer and the oxidant flow channels are closed to form a conduit for supplying oxidant to the MEA layer.
The fuel cells are stacked in sequence, the anode electrode of the fuel cell at one extremity of the stack being electrically connected to the anode terminal, the cathode electrode of the fuel cell at the other extremity of the stack being electrically connected to the cathode terminal, and the anode electrode of each of the other fuel cells in the stack being electrically connected to the cathode electrode of the next adjacent fuel cell. When the anode terminal and cathode terminal are electrically connected through an external load and for each fuel cell hydrogen is supplied to the hydrogen conduit and oxygen is supplied to the oxidant conduit, then in each fuel cell hydrogen moves from the hydrogen flow field through the anode electrode and is ionized at the anode electro-catalyst layer to yield electrons and hydrogen ions, the hydrogen ions migrate through the electrolytic membrane layer to react with oxygen that has moved from the oxidant flow field through the cathode to the cathode electro-catalyst layer and with electrons that have moved from the anode electrode electrically connected to the cathode electrode, thereby to form water as a reaction product, and a useful current of electrons is thereby produced through the load.